如何在laravel 5.4中使用多重关系

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class);
    }
}

class Role extends Model
{
    public function users()
    {
        return $this->belongsToMany(User::class);
    }

    public function permissions()
    {
        return $this->belongsToMany(Permission::class);
    }
}

class Permission extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class);
    }
}

$user = User::find(1);
$roles = $user->roles;

$role = Role::find(1);
$permissions = $role->permissions;

$user = User::find(1);
$user->roles()->attach($roleId);

$role = Role::find(1);
$role->permissions()->detach($permissionId);